Chief Nestel says there were several thefts on those rail lines, but no one used force or a weapon to steal someone else's money or property. The use of force would change the crime’s classification to robbery.

The two lines, which serve an average of 315,000 riders a week, have seen a decline in robberies over the past three months.

Crime data shows there were six robberies on the Market-Frankford Line and one on the Broad Street Line in May and that SEPTA Police made arrests in all of the cases. Those robbery figures dropped to two on the Market-Frankford line and zero on Broad Street in June with no arrests.

“Perhaps there is a general fear among the criminal element of the #cheesesandwich campaign occurring on SEPTA,” he said.

Chief Nestel regularly tweets about criminals on the system and their arrests including the hashtag cheese sandwich, which refers to the food served to suspects in jail.

Other possible reasons?

“Perhaps an epiphany has occurred during which criminals are now escorting little old ladies across streets and engaging in random acts of kindness instead of victimizing SEPTA passengers,” he said. “Perhaps the city's felons have gone on vacation to the French Riviera.”

The chief's jesting predictions aside, he's happy by the drop.

“At this point, I do not have any concrete reason to explain the extremely low number of robberies occurring on a transit system that carries nearly 1 million people per day,” he said. “I can tell you that I am very happy and hope that the criminals continue to do whatever they have been doing this summer.”

The 270 officer-strong force has been focusing on making their presence on the two lines more visible, according to Chief Nestel.

“I can also say that we are being very diligent in apprehending fare evaders and most transit systems have found that persons who don't pay to get on the system are often gaining access to engage in criminal activity,” he said.